1. Field of the Invention
This invention relates to the preparation of an alkoxysilane. More particularly, this invention relates to a continuous process for the production of alkoxysilane wherein HCl gas is continuously removed from the reaction mixture in a distillative reaction zone while the resultant reaction mixture is maintained at its boiling point and liquid alkoxysilane is removed from the sump portion of the distillative reaction zone. This invention is particularly directed to a continuous process for the production of alkoxysilane where alkoxysilane is obtained in substantially quantitative amounts in liquid form.
2. Discussion of the Prior Art
The esterification of chlorosilanes is performed in accordance with the following equation when SiHCl.sub.3 is the chlorosilane: EQU HSiCl.sub.3 + n R'OH .fwdarw. HSi (OR').sub.n Cl.sub.3.sub.-n + n HCl
In this equation, R' represents an alkyl radical of 1 to 11 carbon atoms, and n can assume values between 1 and 3.
The practical performance of this reaction, and of analogous reactions in which a saturated or unsaturated alkyl group replaces a hydrogen atom, causes difficulties because the hydrogen chloride that forms in large quantity in that case, not only cleaves the alkoxy group to alcohol and chlorosilane but also, particularly in the presence of alkanol, cleaves the hydrogen silane compound thereby yielding hydrogen and forming an alkoxysilane compound and a chlorosilane compound. In addition, the hydrogen chloride, in turn, forms chloroalkanes from alkanols charged. Intermediately there is formed water which, in turn, attacks the chlorosilanes and alkoxysilanes. Unless certain process conditions are maintained, the desired silane ester is usually completely lost due to this secondary reaction.
Consequently, a number of attempts have been made to produce such compounds in a more economical manner. The low rate of formation of the condensate, which encumbered the original batch process and was due to the above-described secondary reactions of the hydrogen chloride with the alcohols used for the esterification, can be largely eliminated by the use of modern batch procedures. However, there are limits to the application of such procedures to large-scale technical production, especially on account of the difficulty of controlling the large quantities of hydrogen chloride in conjunction with sometimes low-boiling starting substances, and the large and steep temperature gradients required both in the reaction chamber and in the exhaust for the safe control of the reaction. A continuous process is therefore to be preferred, if only for reasons of better temperature control.
Thus, continuous processes have already been proposed in which chlorosilanes, in the presence of solvents in some cases, are esterified in the liquid phase, either in a reactor provided with an overflow, borrowed from the simplest batch process, or in a plurality of reactors joined together in series on the basis of the counterflow system. This process, however, has the disadvantage that the hydrogen chloride is removed too slowly and incompletely. This results in the cleavage of ester groups already present and in secondary reactions between the alcohols and the hydrogen chloride with the formation of undesirable hydrolyzates. Another prescribed method involves the esterification of chlorosilanes with alcohols in the gas phase. This method uses temperatures which are above the boiling points of all the substances, i.e., reactants as well as end products.
The latter process, however, has a particularly great disadvantage because the hydrogen chloride present in the system coupled with the elevated temperature employed produces the known secondary reactions at a particularly great speed, especially reverse cleavage, alcohol dehydration and the formation of hydrolyzates.
The special weakness of all the continuous esterification processes described above is the excessively slow and incomplete separation of the hydrogen chloride from the reaction mixture. It has already been proposed, therefore, to blow out the hydrogen chloride by passing inert gases, nitrogen for example, over or through the mixture, in some cases with the aid of a falling film evaporator, in which case an upper temperature limit may not be exceeded. This procedure, however, again has the considerable disadvantage that the volume of the exhaust gas consisting of hydrogen chloride is increased. This makes the evaporation losses due to the partial pressure of the products unacceptably high. The reuse of the hydrogen chloride under such circumstances is therefore virtually impossible. On the other hand, the inert gas treatment does not accomplish a complete separation of the hydrogen chloride from the raw product.
It has therefore become desirable to provide a process for the continuous esterification of chlorosilanes with alcohols wherein hydrogen chloride removal is virtually complete and liquid alkoxysilane is obtained having a low concentration of HCl. It has become particularly desirable to provide a process which yields quantitative amounts of alkoxysilane by esterification of chlorosilanes with alcohol. It is an object of this invention, therefore, to provide such a process.